Pfund telescope

The Pfund telescope, originated by A. H. Pfund, provides another method for achieving a fixed telescope focal point in space regardless of where the telescope line of sight is pointed. This configuration utilizes a two-axis feed flat mirror to reflect starlight into a fixed paraboloid of revolution (paraboloidal) mirror, usually with a horizontal optical axis. The paraboloid focuses through a central hole in the feed flat to a convenient distance behind the flat. No spider vanes or Newtonian secondary fold mirrors are required in this configuration. This eliminates vane diffraction and blockage, as well as secondary mirror scattering and absorption, thus improving image brightness and contrast.

The feed flat is mounted on a two-axis azimuth/elevation mount. The azimuth and elevation drive servos must be continuously controlled as objects move across the sky, using vector addition to calculate the mirror motion in real time. One vector (V₁) is stationary and points from the center of the feed flat to the center of the fixed paraboloid mirror. The other vector (V₂) points from the center of the feed flat to the object to be tracked, which of course moves across the sky in time. The surface normal of the feed flat mirror is the 3D bisector of vectors V₁ and V₂, normalized to unity length. If ("N_k","N_l","N_m") are the instantaneous unit vector components of the mirror surface normal, then the mirror elevation angle is arcsin("N_l"), and the mirror azimuth angle is arcsin ["N_k"/cos("Elevation")] . The field of a Pfund telescope rotates at a nonuniform rate during tracking, precluding it from long-exposure astrophotography unless a derotation control matrix and optics are used to compensate field rotation.

The hole in the front face of a Pfund tracking flat should only be large enough to pass the desired field of view with minimum vignetting (blocking of part of the light from the paraboloid) to minimize central obstruction. The hole through the flat must be conically shaped, opening outward toward the back of the flat with at least a 45° cone, to prevent vignetting of the image by the back of the steering flat at high mirror tilt angles. A cylindrical hole would quickly block light passing through the flat from the primary mirror as the steering flat tilt angle increases.

The front reflective face of the Pfund flat must be polished extremely flat, smooth and zone-free. The flat should ideally be flat to within about 25 nanometers peak-to-valley error. Departures from flatness, from figure error or deflection, or both, rapidly introduce unacceptable astigmatism in the image. The front face should lie precisely in the plane of the elevation rotation axis to minimize the required flat mirror aperture. This creates the need for counterweights extending forward from the mirror cell to balance the load on the elevation servo drive.

The diameter of the Pfund flat is generally larger than the focusing paraboloid, and is a design trade between fully illuminated field of view coverage and flat cost and weight. If the Pfund is intended to provide fully illuminated field coverage at a 90°Flat angle, then the minimum flat diameter must be at least $sqrt\; 2=1.4142$ times the paraboloid diameter. The McDonald Observatory Supernova Search Telescope (SNST) used the Pfund configuration, and its feed flat diameter was 24", while the focusing mirror was an 18" f/4.5 paraboloid. The aperture stop is the rim of the focusing paraboloid, thus the feed flat has to be slightly larger than the on-axis diameter required to maximize illumination over the desired field.

Examples of Pfund telescopes are the [http://isi.ssl.berkeley.edu/system_overview.htm Infrared Spatial Interferometer Array] at the University of California at Berkeley. In addition to the array's website, the instrument is described in Dr. Charles Townes' book, "How The Laser Happened" [Townes, Charles H., "How the Laser Happened", Oxford University Press, 1999, pp. 184-185, ISBN 0-19-515376-6 (pbk).] . The same telescope is briefly described and diagrammed on pages 136-137 in Peter Manly's book "Unusual Telescopes" [Manly, Peter L., "Unusual Telescopes", Cambridge University Press, 1999, pp. 136-137, ISBN 0-521-48393-X (paperback).] .

Apparently unaware of Pfund's telescope design, John O. Fundingsland independently developed the same optical configuration, and published his 4" aperture prototype instrument in an August, 1992 Sky and Telescope article entitled "Easy Viewing with a Fixed Telescope" [Fundingsland, John O., "Easy Viewing with a Fixed Telescope", Sky and Telescope, August 1992, pp. 212-215.] .

The George B. Wren Supernova Search Telescope (SNST) at McDonald Observatory and the new Wren-Marcario Wheelchair Access Telescope (WAT) at the McDonald Observatory Visitor Center (to be operational early 2007) are both based on the Pfund configuration. WAT is unique in that it will employ two 18" f/8 mirrors arranged on a north-south line and facing each other, with the steering flat halfway between. The north 18" mirror covers the northern half-hemisphere of the sky, and the south 18" mirror covers the south sky, thus providing full sky coverage, which is not possible with a single-mirror Pfund. The 24" steering flat and viewing port assembly rotate in azimuth to either mirror. Each half-hemisphere has its own fixed image location. WAT is fully compliant with all Americans with Disabilities Act (ADA) requirements and will provide both fully and differently abled visitors to McDonald Observatory with superb and comfortable viewing. A detailed Wikipedia article on the WAT will be included as soon as the instrument is operational, expected in early 2009.

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